Computer image generators used in simulation and in virtual reality systems have an inherent transport delay due to the finite amount of time taken to perform the various computational algorithms necessary to assemble an image of the virtual environment with proper attributes. The effect of this transport delay on the performance of pilots in flight simulators has been well known for many years and care is taken to minimize such delays in image generation systems specifically designed for flight simulation. A far more obvious effect is seen, however, when image generation systems are coupled to head mounted displays. In these systems, the head position is continually being measured and is used by the image generator to compute the correct scene for the observer's viewpoint (the visual orientation of the display with respect to the virtual environment). If the observer moves his or her head while looking at a stationary image, the image will move in the direction of the head motion for a period of time corresponding to the total transport delay of the system (including head measuring device) and will only regain the correct position once the observer's head is stationary.
This effect detracts considerably from the utility of the head mounted display and can give rise to nausea. The problem and a reasonably effective solution is described by Uwe List in a U.S. Airforce report entitled "Non Linear Prediction of Head Movements for Helmet Mounted Displays" (AFHRL Technical Paper 83-45 December 1983). In this report, List recommends the use of angular acceleration sensors mounted on the helmet to calculate a predicted head position. Welch and Kruk also suggest this solution in "HDTV Virtual Reality" published in Japan Display 1992.
These solutions relate to predicting the visual orientation of the display with respect to the virtual environment for a time in the future approximately equivalent to the present time plus the transport delay of the system. This prediction is accomplished by using measurements of angular head acceleration and/or angular head velocity. The image generator then uses this predicted position to compute the next image. While the prediction of visual orientation or head position can be used in the image generator to greatly reduce the error or discrepancy between the image of the virtual environment being displayed and the correct image of the virtual environment for the actual visual orientation, this technique cannot eliminate such errors completely. Accordingly, it is a primary object of this present invention to provide a relatively inexpensive and simple system to substantially reduce such errors thereby providing a more stable and accurate representation of the virtual environment.